Many optical instruments and devices require a known optical power or wavelength output which remains relatively constant even if the ambient temperature of the instrument changes. When optical sources have variations related to changes in the source temperature, then some method of monitoring the source optical power, wavelength, and/or temperature must be provided in order to maintain a desired optical power and/or wavelength output or to predict the optical output. For many optical sources the output power and/or output wavelength will vary with the source temperature. These variations can be compensated for by open- or closed-loop methods if a method of measuring the source temperature or the output power or wavelength is available. If temperature is measured it is normally important that the temperature measurement device be located as close to the optical source as possible so that the measured temperature accurately indicates the temperature of the optical source. Such proximity is often difficult or even impossible to achieve for small optical devices that are often pre-packaged. The present invention provides a method and apparatus by which the temperature of an optical source can be determined through the measurement of certain electrical parameters of the optical source. This method avoids the need to directly measure the temperature itself (i.e., by using a separate sensor) or the parameter being compensated (e.g., output power or wavelength). One important field of application of this invention is the measurement of optical density in cellular media in bioprocess applications. However, this invention is not limited to optical density measurements or biopharmaceutical applications and is pertinent to any application that requires a source of stable optical output power and/or wavelength over a temperature range.
One application of the present invention outside the biotech arena is in diode pumped solid state lasers. Semiconductor lasers are often employed as pump sources for solid state lasers. In this instance, the semiconductor lasers excite the rare earth ions to an excited state in order for laser action to occur. The absorption features of many rare earth doped crystals (e.g.: Nd: YAG, Nd: YVO4) are fairly narrow and require the semiconductor lasers to be temperature stabilized in order to tune their output spectrum to coincide with the rare earth crystal's absorption spectrum. Once the semiconductor laser's spectrum is tuned to the appropriate wavelength, the temperature typically is monitored using a thermistor and controlled through a feed-back loop in order to keep the output at the proper wavelength. The temperature could also be sensed using the invention disclosed herein.
FIG. 1 shows a block diagram of a prior art, open-loop power correction system based on using a thermistor or similar temperature sensing device to measure the optical source temperature and provide a correction factor for the optical power change with temperature [See U.S. Pat. No. 7,099,251; Maxim Application Note HFAN-09.2]. This method has relatively limited utility since it requires that both the thermistor properties and the optical source power variation with temperature be known.
FIG. 2 shows another prior art method of optical power compensation. The optical power is measured directly and the compensation done as shown in either FIG. 2a) open loop, or FIG. 2b) closed loop [See Sipex Application Note SP8029; Maxim Application Note AN3080, Summit Microelectronics White Paper]. In general, closed loop compensation can be more accurate when the power measurement is accurate because all sources of power variation will be corrected.
FIG. 3 shows another prior art method of optical power correction. Thermal variations in the optical source are sensed and a feedback loop is created to a variable temperature source (typically a thermoelectric cooler “TEC”) [See ILX Application Note #2, Linear Technology Application Note AN89-1]. The feedback loop allows the optical source to be held at a constant temperature so optical power output variations will not occur due to changes in source temperature.
However, directly monitoring the optical power or wavelength is often inconvenient, inaccurate, or both. Additionally, separate thermistors or similar temperature sensitive devices may be difficult or impractical to mount sufficiently proximate to the optical power source to provide an accurate reading of the source temperature. These facts, in conjunction with the fact that using a thermoelectric cooler is often impractical, leads to the need to sense the temperature of the optical source without requiring a separate sensor.
Semiconductor physics provides a direct relationship between temperature change and junction forward voltage drop. FIG. 4 shows a semiconductor diode junction with current and voltage indicated for forward bias conditions. The equation for the current through the junction is given by Equation (1). In Equation (1) I is the current through the junction, V is the voltage across the junction, IS is the junction saturation current, and VT is defined in Equation (3).I=IS(eV/VT−1)  Equation (1)
A semiconductor diode also contains a finite resistance resulting from the ohmic contacts on the semiconductor and any wire length resistance from connections to the junction. Because the voltage drop across the resistance obeys Ohms law, namely V=I*R, the voltage drop across the entire device can be calculated. Voltage drops across the junction and the wire length resistance will be referred to as the “diode voltage drop”. Voltage drop across just the junction will be referred to as the “junction voltage drop”. For any practical operating temperature range (typically between 273 and 325 K), both these resistances can be considered to be independent of temperature.
Under forward bias conditions, where most optical sources (such as those used in a cell density monitor) will operate, the exponential term in Equation (1) dominates and one can write the junction forward bias voltage as shown in Equation (2).V=VT ln(I/IS)  (Equation (2)
Equation (2) shows that if I is held constant then the junction forward voltage is a function of VT and IS. As VT increases the junction forward voltage increases and as IS increases the junction forward voltage decreases. Temperature changes in VT and IS will cause corresponding changes in V, so the temperature sensitivities of both VT and IS need to be understood.
The VT term in Equation (1) is defined in Equation (3).VT=nkT/q  Equation (3)
In Equation (3) n is the junction ideality factor (which is approximately unity), k is Boltzmann's constant, and T is the temperature in degrees Kelvin (See Sze, Physics of Semiconductor Devices, John Wiley & Sons, 1981, pp 63-132). From the definition of VT we see that dVT/dT is positive. By inspection Equation (3) also shows that an increase in temperature causes an increase in VT.
The IS term in Equation (1) is defined in Equation (4).IS=BT3/2e−Eg/nkT  Equation (4)
In Equation (4), B is a physical constant of the particular semiconductor material, and Eg is the band gap energy of the semiconductor material. By inserting Equations (3) and (4) into Equation (2) we get Equation (5).
                    V        =                                            nkT              q                        ⁢                          ln              ⁡                              (                                  I                                      BT                                          3                      /                      2                                                                      )                                              +                                    E              g                        q                                              Equation        ⁢                                  ⁢                  (          5          )                    
For small changes in temperature, such as a ˜20° C. variation about a mean temperature of 25° C., the constant term in Equation (5) can be ignored and the variation with forward junction voltage versus temperature will be approximated by Equation (6). For a temperature range of 10° C. to 50° C., Equation (6) reduces to approximately −2.3 mV/° C. for a silicon diode with an ideality factor of unity (Sze, op. cit. supra).
                                          ⅆ            V                                ⅆ            T                          ≈                              nk            q                    ⁢                      ln            ⁡                          (                              I                                  BT                                      3                    /                    2                                                              )                                                          Equation        ⁢                                  ⁢                  (          6          )                    
Based on Equation (5), which embodies principles of semiconductor physics, we have ascertained that the voltage across the diode junction can serve as a very accurate indicator of relative temperature shift in an optical power source.
Although the use of the diode forward bias voltage has apparently previously been utilized for temperature compensation using a transistor proximate to the diode (See U.S. Pat. No. 5,397,933), using the diode forward bias voltage across an optical device as a direct indicator of the optical source's own temperature has not heretofore been applied to optical sources such as vertical cavity surface emitting lasers (VCSELs) or LEDs. Historically, temperature induced variations in the optical output power of semiconductor laser devices have been mitigated by using methods such as closed-loop correction involving direct measurement of the optical power, open-loop correction by using a separate temperature transducer near the optical device, or by a feedback system that controls the device temperature, e.g. a resistive temperature device (RTD) connected to a thermo-electric cooler. For instance, in the field of diode pumped solid state lasers, the laser is used to provide optical excitation energy for a rare earth ion doped host crystal. The rare earth ions typically have narrow absorption features, which require the laser diode pumps sources to be temperature stabilized, as the wavelength of emission and the power of the laser diodes changes as a function of temperature.
Our invention exploits the electrical response of a laser or LED itself to changes in its temperature, in order to compensate for changes in the optical output. By combining the principles of semiconductor physics with electrical measurements of the laser junction voltage, we have discovered that one can compensate for temperature variations in the optical power generation of the optical device. This ensures that the temperature measurement is closely correlated with the optical power source and minimizes the number of components required in a practical implementation. It is also important to measure the voltage across the diode accurately. FIG. 5 shows a prior art schematic of a differential amplifier used to measure the voltage across the diode [Horowitz and Hill, The Art of Electronics, Cambridge University Press, 1980, pp. 176-185].
Finally, it is important to note that the present invention applies, but is not limited, to the optical loss measurement shown in FIG. 6. However, a particularly advantageous use of the present invention does apply to the optical loss measurement as shown in FIG. 6. The optical source is driven by some input signal, typically current, and produces an output power which is primarily a function of the input signal and the ambient temperature PS(S,T). As the optical power travels through an optically dense sample medium the transmitted power measured by the photodetector is affected by one or more mechanisms such as absorption, scattering, or reflection in the sample medium. The result is a power at the detector, Pd(S,T), which is less than or equal to the source power. An “absorbance unit” may be associated with the sample medium defined by “optical loss” or “optical density” such that the “absorbance unit” or AU is given by log10(PS/Pd).